1. Field Of The Invention
This invention relates to sub-surface release plugs used in cementing of the outer casing annulus of a wellbore, and more particularly, to a subsurface release plug assembly having high strength non-metallic components for allowing operation under relatively high pressures and temperatures and also allowing easy drilling thereof, such as with a polycrystalline diamond compact (PDC) drill bit.
2. Description Of The Prior Art
Typically, sub-surface release plugs positioned in the upper portion of a well casing below a casing hanger and attached to the lower end of a drill string are used in cementing operations for cementing a casing annulus adjacent a shoe joint. The construction and use of such plugs is disclosed in U.S. Pat. Nos. 4,809,776 and 4,934,452, both of which are assigned to the assignee of the present application, and both of which are hereby incorporated by reference. Another prior art sub-surface release plug assembly is disclosed in Halliburton Services Sales & Service Catalog No. 43, pages 2424-2426.
Typically, a bottom plug of the assembly is released, and cement in pumped into the casing above the bottom plug, forcing the bottom plug downwardly until it comes to rest at the upper end of the shoe joint. The bottom plug seals against the inner surface of the casing so that mud below the bottom plug and cement above the bottom plug are not mixed. Once the bottom plug has reached its lowermost position, a passageway in the bottom plug is opened to allow cement to pass therethrough. The cement then passes through a float collar and/or float shoe and an opening at the lower end of the shoe joint in the casing annulus. A valve in the float collar and/or float shoe prevents reverse movement of the cement through the casing.
When the proper amount of cement has been introduced into the casing and drill string, a releasing dart or drill pipe plug is dropped into the drill string. The releasing dart engages a latching mechanism above the top plug, thus closing off the central opening of the top plug and releasing it from the drill string. The fluid pumped into the drill string forces the top plug, and the dart or drill pipe plug latched thereto, down toward the bottom plug, forcing the cement through the shoe joint. The top plug stops when it contacts the bottom plug.
Once the cement has set, the top and bottom plugs are drilled out of the casing. Mating teeth on the upper and lower plugs prevent relative rotation therebetween so the top plug does not merely rotate when contacted by a drill bit. Most prior art sub-surface plug assemblies use metallic components, and the drill bits used to drill the components out of the casing must be adapted for cutting such materials. Typically, standard "tri-cone" rotary drill bits are used with appropriate loading applied thereto. Such prior drillable devices have worked well, but drilling out iron components requires certain techniques. Ideally, the operator employs variations in rotary speed and bit weight to help break up the metal parts and reestablish bit penetration should bit penetration cease while drilling. A phenomenon known as "bit tracking" can occur, where the drill bit stays on one path and no longer cuts into the plug assembly. When this happens, it is necessary to pick up the bit above the drilling surface and rapidly recontact the bit with the plug and apply weight while continuing rotation. This aids in breaking up the established bit pattern and helps to reestablish bit penetration. If this procedure is used, there are rarely problems. However, operators may not apply these techniques or even recognize when bit tracking has occurred. The result is that drilling times are greatly increased because the bit merely wears against the surface of the plug rather than cutting into it to break it up.
While metallic components, such as cast iron, may be necessary for some pressures and temperatures, many wells experience less severe conditions. This includes most wells cemented. Thus, the heavy-duty metal construction of previous sub-surface plug assemblies is not necessary for many applications, and if cast-iron components can be eliminated or minimized, the potential drilling problems resulting from bit tracking might be avoided as well.
The sub-surface release plug of the present invention solves this problem by providing an apparatus wherein at least some of the components are made of non-metallic materials, and in particular, high strength plastic. Molding of the plastic plugs also eliminates some of the machining necessary on metallic components. Such plastic components are more easily drilled than cast iron, and new drilling methods may be employed which use alternative drill bits, such as polycrystalline diamond compact (PDC) bits, or the like, rather than standard tri-cone bits.
Sub-surface release plugs have been made of non-metallic materials, such as plastic, and the drilling problems associated with plug assemblies using metallic components avoided because the plastic is easier to drill. However, the use of such plastic plugs have been limited in the past to relatively low pressure and temperature conditions in the well casing. Prior art sub-surface release plug assemblies utilizing plastic components have been limited to pressures in the range of about 3,500 to about 5,000 psi and also limited to maximum temperatures of about 300.degree. F. The present invention improves on these prior art subsurface release plug assemblies by utilizing a high strength plastic material which allows pressure up to about 10,000 psi and temperatures in the range of about 300.degree. F. to about 400.degree. F. Of course, the plastic materials used in the present invention may also be used at temperatures and pressures below these levels.